1. Field of the Invention
This invention relates to a system and method for inspecting inner surfaces of cylindrical parts, such as cylinder liners for internal combustion engines.
2. Description of the Prior Art
Cylindrical parts, such as heavy-duty liners for internal combustion engines, are inspected manually by humans for subtle defects, such as cracks and porosity defects, in an effort to avoid shipping defective parts to a customer. The costs associated with this manual inspection process add up to hundreds of thousands of dollars each year. Further, a single human inspector is typically required to inspect thousands of cylindrical parts each day. The cylindrical parts are heavy, and subtle defects, such as those less than 200 microns, as well as defects located near the center of the cylindrical parts, are hardly visible to the naked eye. Thus, chances of overlooking defects due to physical or mental fatigue, or inability to see the defects, and consequently shipping a defecting part to a customer are high.
In attempt to reduce human error, automated surface inspection techniques have been developed. An example of an automated technique is disclosed in U.S. Patent Application Publication No. 2007/0132990. Machine vision techniques have also been employed for inspection of cylindrical parts, including inspection of both the inner and external surface of the cylindrical part. However, the techniques used to inspect the inner surface either require the camera to be disposed inside the cylindrical part or require a conical mirror. The first technique requires on-axis lighting and the camera and lens are subject to size restraints. Thus, images acquired typically have low resolution and distortion. In addition, the position of the camera and light source must be adjusted numerous times to acquire images of the entire inner surface, which is burdensome and time consuming. The second technique requires an appropriately sized conical mirror for every cylindrical part inspected. Further, the conical mirror typically causes distortion in the image due to compressed pixels.